Polyadenylated RNA from cells transformed by transfection with subgenomic proviral DNAs of ml and HT-1 Moloney murine sarcoma virus (M-MSV) were analyzed for mos containing RNA. Cells transformed by subgenomic proviral DNAs containing a 5' LTR relative to the v-mos sequence were found to exhibit identical RNA patterns when Northern blots of these RNAs were analyzed with probes representing mos and the unique 5' region of the proviral LTR (U5). These RNAs did not contain sequences hybridizable to the probe prepared from the unique 3' region of the LTR (U3). The size of the transcripts containing mos were in excess of the DNA used in the transfection and imply the involvement of host or carrier DNA sequences in these transcripts. Poladenylated RNA, isolated from cells transformed by transfection with a subgenomic proviral DNA in which the LTR was 3' relative to v-mos, showed identical RNA patterns with U3 and mos probes, but not with U5. Analysis of polyadenylated RNA from cells transfected with an LTR: c-mos hybrid molecule showed several mos-specific RNAs and demonstrate that c-mos is activated for expression by the proviral LTR sequence.